Chemical pressure-induced topological order: the case of SrCd₂Sb₂

Helicity-dependent THz emission measurements have been performed on both SrCd₂As₂ and SrCd₂Sb₂ single crystals. This technique provides a contact-free method to study spin-polarized photocurrent from topological surface states due to circular photogalvanic effect (CPGE). In normal incident case, both SrCd₂As₂ and SrCd₂Sb₂ single crystals shows almost zero CPGE term by fitting of quarter waveplate alpha-scan results. In the case of 45 degree incidence, SrCd₂Sb₂ single crystals show significant CPGE term indicating existence of non-trivial topological surface state exists on the surface. The current results agree with theoretical calculations and indicate non-trivial topological phase existing SrCd₂Sb₂ single crystals.